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| United States Patent |
5,255,113
|
|
Yoshikawa
, et al.
|
October 19, 1993
|
Pos-objective type optical scanner
Abstract
A post objective type optical scanner for having a light source for
emitting a light beam having a first width in the scanning direction and a
second width in the sub-scanning direction, a single lens having an
incident surface and an emergent surface and including a scanning optical
system having a first light source side principal point, and a
sub-scanning optical system having a second light source side principal
point, for converting a light beam emitted from the light source having a
scanningwise width and a subscanningwise width, the scanning optical
system converting the first width of the light beam into the scanningwise
of the scanning beam while the sub-scanning optical system converting the
second width of the light beam into the subscanningwise width of the
scanning beam. The distance of the first light source side principal point
from the light source is greater than the distance of the second light
source side principal point from the light source. The incident surface of
the single lens is toric, having an aberration correction surface for the
sub-scanning optical system, while the emergent surface is toric or
cylindrical, having an aberration correction surface for the sub-scanning
optical system.
| Inventors:
|
Yoshikawa; Motonobu (Nishinomiya, JP);
Yamamoto; Yoshiharu (Toyonaka, JP)
|
| Assignee:
|
Matsushita Electric Industrial Co., Ltd. (Osaka, JP)
|
| Appl. No.:
|
856642 |
| Filed:
|
March 24, 1992 |
Foreign Application Priority Data
| Current U.S. Class: |
359/196; 359/207; 359/216 |
| Intern'l Class: |
G02B 026/08 |
| Field of Search: |
359/196,205,206,207,216,217,218,219
|
References Cited
U.S. Patent Documents
| 4674825 | Jun., 1987 | Tateoka et al. | 359/218.
|
| Foreign Patent Documents |
| 0286368 | Oct., 1988 | EP.
| |
| 2917221 | Nov., 1979 | DE.
| |
| 59-154403 | Sep., 1984 | JP.
| |
| 59-216122 | Dec., 1984 | JP.
| |
| 61-254915 | Nov., 1986 | JP.
| |
| 1-109317 | Apr., 1989 | JP.
| |
Primary Examiner: Ben; Loha
Attorney, Agent or Firm: Stevens, Davis, Miller & Mosher
Claims
What we claim is:
1. A post-objective type optical scanner comprising:
a light source for emitting a light beam having a first width in a scanning
direction and a second width in a sub-scanning direction;
a single lens having a light source side incident surface and an image
plane side emergent surface, for converting said light beam into a
scanning beam having a scanningwise width in said scanning direction and a
subscanningwise width in said sub-scanning direction, said single lens
comprising a scanning optical system having a first light source side
principal point, for converting said first width into said scanningwise
width, and a sub-scanning optical system having a second light source side
principal point, for converting said second width into said
subscanningwise width; whereby the distance of said first light source
side principal point to said light source is greater than the distance of
said second light source side principal point to said light source;
a means for scanning said scanning beam from said single lens on a scanning
plane; and
a correction lens arranged between said means for scanning and said
scanning plane and having a sub-scanning focal distance which varies in
said scanning direction from the center to each of both side ends of said
correction lens, for focusing said scanning beam onto said scanning plane.
2. An optical scanner as set forth in claim 1, wherein said incident
surface is a first toric surface which is concave in said scanning
direction but convex in said sub-scanning direction, and said emergent
surface is a second toric surface which is convex in said sub-scanning
direction.
3. An optical scanner as set forth in claim 2, wherein said first toric
surface has an aberration correcting surface for said sub-scanning optical
system, and said second toric surface has an aberration correcting surface
for said scanning optical system.
4. An optical scanner as set forth in claim 3, wherein each of said
aberration correcting surfaces is defined by a series expansion including
a member having an order higher than a biquadratic order.
5. An optical scanner as set forth in claim 1, wherein said means for
scanning includes a deflector comprising a deflecting surface which is
cylindrical.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical scanning system and, more
particularly, to a post-objective type optical scanner in which a laser
beam converted into a pencil of rays is deflected by a polygonal mirror.
Still more particularly, the present invention is concerned with a
post-objective type optical scanner which utilizes an anamorphic single
lens.
2. Description of the Related Art
Post-objective type optical scanners employ a laser beam source which emits
a pencil of rays and a polygonal mirror which deflects the pencil of rays.
Because of compact construction and low costs of production, this type of
optical scanner has been studied, developed and put to commercial
production in recent years.
In general, an optical scanner has a main scanning optical system which
conducts scanning in the direction of the major axis of an elliptical
cross-section of a beam from a laser diode, and a sub-scanning optical
system which performs scanning in the direction of the minor axis of the
elliptical cross-section.
In a post-objective type optical scanner of the present invention, the
focal distance of the main scanning optical system is more than 10 times
as large as that of the sub-scanning optical system. Therefore, in order
to equalize the dimensions of the beam spot on the image surface both in
the directions of main scan and sub-scan, it is necessary that the
aperture stop disposed on the incident side of the polygonal mirror has a
rectangular or an elliptical form which is 10:1 or greater in terms of the
ratio between the width as measured in the direction of main scan and the
width as measured in the direction of sub-scan. For this reason, a mere
collimation of a semiconductor laser beam does not provide a required
level of rate of utilization of light. A solution to this problem is to
use prisms. FIG. 5 illustrates an optical scanner which uses prisms. This
optical scanner has a semiconductor laser 41, a collimator lens 42, prisms
43, 44, an aperture stop 45, a convergent lens 46, a cylindrical lens 47,
a mirror 48, a polygonal mirror 49 having cylindrical surfaces, a
compensating lens 50, and a photosensitive drum 51. As will be seen from
FIGS. 6a and 6b, the laser beam from the semiconductor laser 41, which is
disposed such that the direction of greatest divergence angle of beam
coincides with the direction of main scan, is changed into a collimated
beam having an elliptical intensity distribution of about 3:1 in terms of
the ratio between the size in the direction of main scan and the size in
the direction of sub-scan. The beam is then transmitted through prisms 43,
44 which are arranged to contract the beam spot size only in the direction
of sub-scan, so that the beam is changed into a collimated beam having an
elliptical form of intensity distribution of about 10:1 in terms of the
ratio between major and minor-axes. The beam then impinges upon the
cylindrical lens 47 through the aperture stop 45 and the convergent lens
46.
A lens which plays the double role of a collimator lens and prisms,
intended for use in optical disk systems, has been disclosed in Japanese
Patent Unexamined Publication No. 61-254915.
The use of prisms, however, is disadvantageous in that the cost is raised
and the size of the system inevitably is large due to intricacy of
arrangement of optical components. The lens shown in Japanese Patent
Unexamined Publication No. 61-254915 also has drawbacks in that it
exhibits relatively large residual spherical aberration because its
surfaces are toric surfaces which are represented simply by radii of
curvature, and in that this lens does not offer any advantage when applied
to a post-objective type optical scanner since it is a lens intended for
changing a beam having a flattened cross-section into a circular
cross-section.
SUMMARY OF THE INVENTION
Accordingly, an object of the present invention is to provide a
post-objective type optical scanner employing an integral anamorphic
single lens which can play the roles of the collimator lens, a pair of
prisms and the cylindrical lens of the optical system explained before in
connection with FIG. 5, that is which can eliminate the necessity of these
several components.
The anamorphic single lens of the present invention is constructed to
comprise a main scanning optical system and a sub-scanning optical system
which have different magnitudes of refractive power in main and
sub-scanning directions, respectively, so as to change a beam from a beam
source having different angles of beam divergence in the main and
sub-scanning directions into a collimated beam, a convergent beam or a
divergent beam. Representing the distance between the beam source and the
incident-side principal position of the main scanning optical system by S,
and the distance between the beam source and the incident-side principal
point of the sub-scanning optical system by S', the above-mentioned single
lens is determined to meet the condition of S>S'.
In view of the preferred embodiment of the present invention, the first
surface, i.e., the incident surface of this single lens is a toric surface
which is concave in the direction of the main scan and convex in the
direction of the sub-scan and which has terms of development of quartic
and higher orders contributing to aberration correction only in the
direction of the sub-scan. The second surface, i.e., the emergent surface,
of this single lens is a toric surface convex in the direction of the main
scan, or a cylindrical surface, having terms of development of quartic and
higher orders contributing to aberration correction only in the direction
of the main scan. The radii of curvatures of the incident and emergent
lens surfaces in each scanning direction, as well as the higher-order
developed terms, are determined in accordance with factors such as the
rate of utilization of light required for the whole optical system,
imaging positions in both scanning directions and an optical performance
to be attained.
Thus, the radii of curvatures of the lens surfaces in each scanning
direction are determined to meet requirements for the rate of utilization
of light and the imaging position in each scanning direction, while
satisfying the above-mentioned condition of S>S'. As the emergent side of
this single lens, a light beam is wide-spread or diverged in the direction
of main scan and contracted or converged in the direction of sub-scan,
thereby obtaining an intensity distribution of an elliptical form which
has a major axis in the direction of the main scan, thus improving the
rate of utilization of light. At the same time, the beam can be focused
correctly at a required imaging position through the whole optical system.
In the single lens used in the present invention, as stated before, the
first surface as the incident surface of this single lens is a toric
surface which is concave in the direction of the main scan and convex in
the direction of the sub-scan and which has terms of development of
quartic or higher orders contributing to aberration correction only in the
direction of the sub-scan, whereas, the second surface, i.e., the emergent
surface, is a toric surface convex in the direction of the main scan, or a
cylindrical surface, having terms of development of quartic or higher
orders contributing to aberration correction only in the direction of the
main scan. With these features, this single lens can cope with demands for
optical performance to be attained by the whole optical system.
Furthermore, by using the anamorphic single lens having the above-described
features, it is possible to realize an inexpensive and compact optical
scanner.
Therefore, a post-objective type optical scanner, employing such anamorphic
single lens, when employed in an image forming apparatus, reduces the size
and cost of such image forming apparatus.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a perspective view of an example of anamorphic single lens used
in an embodiment of a post-objective type optical scanner in accordance
with the present invention;
FIG. 2a is a top plan view of an optical system employing the anamorphic
single lens shown in FIG. 1;
FIG. 2b is a side elevational view of the optical system shown in FIG. 2a;
FIG. 3 is a perspective view of an embodiment of the post-objective type
optical scanner of the present invention;
FIG. 4 is an illustration of an image forming apparatus incorporating the
optical scanner of the present invention;
FIG. 5 is an illustration of a conventional optical scanner;
FIG. 6a is a top plan view of the conventional optical scanner shown in
FIG. 5; and
FIG. 6b is a side elevational view of the optical scanner shown in FIG. 6a.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
A preferred embodiment of the present invention will be described with
reference to the accompanying drawings.
FIG. 1 shows an anamorphic single lens suitable for use in a post-objective
type optical scanner of the present invention. The single lens 1 has a
first surface 2 adjacent to a light source, i.e., an incident surface, and
a second surface 3 adjacent to an image plane, i.e., an emergent surface.
This single lens is designed and formed so as to have a main-scanning
optical system and a sub-scanning optical system. The main scanning
optical system is provided by specific radii of curvatures of lens
surfaces as shown in FIG. 2a and converges as incident beam in the manner
shown in the same Figure. The sub-scanning optical system is provided by
specific radii of curvatures of lens surfaces as shown in FIG. 2b and
converges an incident beam in the manner shown in the same Figure. The
optical arrangement shown in FIGS. 2a and 2b has, in addition to the
anamorphic single lens 1, a semiconductor laser 4 and an elliptical
aperture stop 12. Numeral 5 denotes a laser emitting point as the object
point, 6 denotes the position of the incident-side principal point of the
main scanning optical system, 7 denotes the position of the imaging-side
principal point of the main scanning system, 8 denotes the position of the
incident-side principal point of the sub-scanning system, and 9 denotes
the position of the imaging-side principal point of the sub-scanning
system. Numerals 10 and 11 respectively indicate the focal positions of
the main and sub-scanning systems.
The first surface 2 of the anamorphic lens 1 is curved at a radius R.sub.1H
in the direction of the main scan and at a radius R.sub.1V in the
direction of the sub-scan. This surface is a toric surface which has
developed terms of quartic and higher orders which contribute to
correction of aberration only in the sub-scanning direction. The second
surface 3 of the anamorphic lens 1 is curved at a radius R.sub.2H in the
direction of the main scan and at a radius R.sub.2V in the direction of
the sub-scan. This surface is a toric surface which has developed terms of
quartic and higher orders which contribute to correction of aberration
only in the main scanning direction The geometries of these toric
surfaces, in terms of amounts of sag from the apices of the toric
surfaces, are represented by the following developed equations (1) and (2)
based on an X-Y-Z coordinate system as shown in FIGS. 2a and 2b.
Similarly, geometries of the second surface is expressed by the equations
(3) and (4).
##EQU1##
In the higher-order developed equation for aberration correction in the
main scanning direction, K.sub.H represents a conic coefficient, and
A.sub.H, B.sub.H, C.sub.H and D.sub.H represent coefficients of higher
orders. In the higher-order developed equation for aberration correction
in the sub-scanning direction, K.sub.V represents a conic coefficient, and
A.sub.V, B.sub.V, C.sub.V and D.sub.V represent coefficients of higher
orders.
Referring to FIGS. 2a and 2b, the distance between the laser emission point
5 and the position 6 of the incident-side principal point of the main
scanning system is represented by S, and the distance between the laser
emission point 5 and the position 8 of the incident-side principal point
of the sub-scanning system is represented by S'. The central lens
thickness is expressed by TH. The distance between the laser emission
point 5 and the first surface is expressed by ff. The distance between the
focal position 10 of the main scanning optical system and the second
surface is represented by bf.sub.H, and the distance between the focal
position 11 of the sub-scanning optical system and the second surface is
represented by bf.sub.V. Practical numerical values of these factors are
shown in Tables 1, 2, 3 and 4. The design wavelength is 788 nm, while the
vitreous material is SF 8.
TABLE 1
______________________________________
R.sub.1H
-2.7029 R.sub.1V
2.2453 TH 8
R.sub.2H
-4.3146 R.sub.2V
25.0385 ff 3.77
K.sub.H
-3.97930E - 01
K.sub.V -7.34099E - 01
A.sub.H
2.18881E - 05
A.sub.V -1.78496E - 02
B.sub.H
7.03154E - 07
B.sub.V -7.66638E - 04
C.sub.H
-1.89559E - 08
C.sub.V 1.67513E - 02
D.sub.H
1.34658E - 09
D.sub.V -1.75090E - 02
S 11.82 S' 3.36
bf.sub.H
137.2659 bf.sub.V
70
______________________________________
TABLE 2
______________________________________
R.sub.1H
-3.2096 R.sub.1V
2.2351 TH 9
R.sub.2H
-4.7433 R.sub.2V
54.8385 ff 3.6
K.sub.H
-4.04325E - 01
K.sub.V -2.47634E + 00
A.sub.H
1.48856E - 05
A.sub.V -1.23479E - 04
B.sub.H
3.57869E - 07
B.sub.V -2.88800E - 03
C.sub.H
-1.01931E - 08
C.sub.V 3.19496E - 03
D.sub.H
4.03148E - 10
D.sub.V 1.15544E - 01
S 11.88 S' 3.39
bf.sub.H
137.2659 bf.sub.V
70
______________________________________
TABLE 3
______________________________________
R.sub.1H
-3.3324 R.sub.1V
2.1351 TH 10
R.sub.2H
-5.0391 R.sub.2V
-57.8469 ff 3.2
K.sub.H
-4.07125E - 01
K.sub.V -2.68763E + 00
A.sub.H
1.28650E - 05
A.sub.V -8.50962E - 04
B.sub.H
3.99698E - 07
B.sub.V -5.21152E - 03
C.sub.H
-1.68494E - 08
C.sub.V 7.76519E - 03
D.sub.H
5.20370E - 10
D.sub.V 3.25931E - 01
S 11.80 S' 3.43
bf.sub.H
137.2659 bf.sub.V
70
______________________________________
TABLE 4
______________________________________
R.sub.1H
-2.5949 R.sub.1V
1.8181 TH 11
R.sub.2H
-5.1168 R.sub.2V
-13.1981 ff 2.39
K.sub.H
-4.07660E - 01
K.sub.V -3.19308E + 00
A.sub.H
1.30857E - 05
A.sub.V -3.38390E - 03
B.sub.H
3.54225E - 07
B.sub.V -2.01399E - 02
C.sub.H
-1.45443E - 08
C.sub.V 1.83038E - 02
D.sub.H
4.74867E - 10
D.sub.V 4.37069E + 00
S 11.36 S' 3.52
bf.sub.H
137.2659 bf.sub.V
70
______________________________________
The effect of the anamorphic lens having these features will be described
with reference to FIGS. 1 and 2a and 2b.
Referring to FIGS. 2a and 2b, a semiconductor laser 4 is disposed such that
the greater divergence angle coincides with the direction of the main
scan. The divergence angle of beam in the main scanning direction is
expressed by a (deg), while the divergence angle of beam in the
sub-scanning direction is represented by b (deg). Thus, the condition of
a>b is met. In such a case, the laser beam which is going to impinge upon
the lens has an elliptical intensity distribution which is expressed by
tan (a/2):tan (b/2) in terms of the ratio between the size in the main
scanning direction and the size in the sub-scanning direction. The beam
from the semiconductor laser 4, when passing through the first surface of
the lens which is concave in the main scanning direction, is diverged in
the main scanning direction and is converged by the second surface of the
lens which is convex in the main scanning direction, so as to be focused
on the focal position 10 demanded by the whole scanning system. The beam
is converged in the sub-scanning direction by the first surface of the
lens as it passes through the first surface which is convex in the
sub-scanning direction and is further converged by the second surface of
the lens so as to be focused at the focal position 11 demanded by the
whole optical system. The intensity distribution of the beam immediately
after emerging from the lens is expressed by the ratio md:sd, where md
corresponds to the size in the main scanning direction and sd corresponds
to the size in the sub-scanning direction.
As stated above, the sizes md and sd are respectively represented as
follows:
md=tan (a/2).times.S
sd=tan (b/2).times.S'
Assuming that the aforementioned beam sizes a and b are respectively given
by a=30 and b=10 in the case of the example given in Table 1, the values
md and sd are respectively calculated as md=3.17 and sd=0.29. When the
condition S>S' is met, it is possible to obtain an elliptical intensity
distribution pattern which is further elongated in the main scanning
direction to a major-to-minor axis ratio at about 11:1. Consequently, loss
of light which is caused when the beam passes through the elliptical
aperture stop 12 is considerably reduced, so that a rate of utilization of
light which is demanded by the whole scanning optical system can be
obtained. This advantageous effect is obtainable also with other examples
shown in Tables 2 to 4.
The spherical aberration in the main scanning direction has been corrected
by the second surface of the lens, while the spherical aberration in the
sub-scanning direction has been corrected by the first surface of the
lens.
The operation of the optical scanner of the invention will be described
with reference to FIGS. 3 and 4.
FIG. 3 is an illustration of an optical scanner employing the anamorphic
single lens described hereinbefore. Referring to this figure, the light
beam from a semiconductor laser 21 is made to pass through the anamorphic
single lens which is in this case denoted by 22, so as to be changed into
a beam having an elliptical intensity distribution pattern, while being
converged in the main scanning direction. The beam then passes through an
elliptical aperture stop 23 and, after being reflected by a mirror 24, is
converged in the sub-scanning direction in a region near to one of
cylindrical reflection surfaces of the polygonal mirror 25. The polygonal
mirror 25 rotates about its axis so as to deflect the light impinging
thereon. The deflected light is then focused on the surface of a
photosensitive drum through a compensating lens 26 thereby to scan the
surface of the photosensitive drum 27 with a simultaneous correction of
curvature of field in the main scanning direction. The compensating lens
26 is disposed in such a manner as to provide a conjugate opto-geometrical
relation between the point of deflection and the scanned point on the
photosensitive drum 27 in the sub-scanning direction, thereby effecting
correction of any tilt of surface of the cylindrical polygonal mirror. At
the same time, the refractive power of the compensating lens 26 in the
sub-scanning direction is progressively decreased in the direction of main
scan from the center towards the peripheral region of this lens, thereby
to effect a correction of curvature of field in the sub-scanning
direction. Correction of f.theta. characteristic can be effected by
converting electrical clock in the signal output into the scanning
position.
According to the present invention, it is possible to obtain a
post-objective type optical scanner which is small in size, low in price
and excellent in resolution, by virtue of the use of the above-described
anamorphic single lens together with a compensating lens and a polygonal
mirror having cylindrical surfaces.
FIG. 4 shows an image forming apparatus which incorporates the
post-objective type optical scanner embodying the present invention.
Referring to this Figure, the image forming apparatus has the following
components: a photosensitive drum 31 on which the states of charge are
changeable upon irradiation with a light beam; a primary charger which
deposits electrostatic ions on the surface of the photosensitive drum 31
so as to uniformly charge the drum surface; an optical scanner which is of
the same type as the described embodiment and adapted for writing
information in the form of electrostatic latent image on the surface of
the photosensitive drum 31; a developing unit 34 for developing the latent
image by depositing charged toner; a transfer charger 35 for transferring
the toner image from the surface of the photosensitive drum 31 to a sheet
of paper; a cleaner 36 for removing any residual toner from the drum
surface; a fixing unit 37 for fixing the transferred toner image onto the
paper; and a sheet feeder cassette 38.
It will be seen that, by using the post-objective type optical scanner of
the invention, it is possible to obtain a small-sized and inexpensive
image forming apparatus.
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